The preparation of block copolymers is well known. In a representative synthetic method, an initiator compound is used to start the polymerization of one monomer. The reaction is allowed to proceed until all of the monomer is consumed, resulting in a living homopolymer. To this living homopolymer is added a second monomer that is chemically different from the first. The living end of the first polymer serves as the site for continued polymerization, thereby incorporating the second monomer as a distinct block into the linear polymer. The block copolymer so grown is living until terminated.
Termination converts the living end of the block copolymer into a non-propagating species, thereby rendering the polymer non-reactive toward monomer or coupling agent. A polymer so terminated is commonly referred to as a diblock copolymer. If the polymer is not terminated the living block copolymers can be reacted with additional monomer to form a sequential linear tri-block copolymer. Alternatively the living block copolymer can be contacted with multifunctional agents commonly referred to as coupling agents. Coupling two of the living ends together results in a linear triblock copolymer having twice the molecular weight of the starting, living, diblock copolymer. Coupling more than two of the living diblock copolymer regions results in a radial block copolymer architecture having at least three arms.
One of the first patents on linear ABA block copolymers made with styrene and butadiene is U.S. Pat. No. 3,149,182. Various block copolymers and processes for making them have been proposed over the years. Studies of such polymers and their morphology in the past have shown that normally there is a phase transition from a spherical morphology to a cylindrical morphology at about 17 wt. % styrene and cylindrical to lamellar morphology at about 32 wt. % styrene. While spherical and cylindrical morphologies still have a continuous rubber matrix that makes them soft and elastic, lamellar morphologies are continuous in both the elastic and rigid phases, thus becoming a plastic. This means that it has not been possible in the past to obtain a truly elastomeric block copolymer having a styrene content of as high as 70 wt. %. In the past, block copolymers with styrene contents greater than about 35 wt. % were merely high impact polystyrene plastics. Higher styrene content block copolymer elastomers have been made by dispersing styrene monomers in the elastomer phase, but these increase the glass transition temperature (Tg) of the elastomer and represent synthetic challenges in manufacturing.
Theoretical [S. T. Milner, Macromolecules 27, 2333-2335 (1994).] and experimental [D. J. Pochan et. al., Macromolecules 29, 5091-5098 (1996)] investigations have been reported in the literature on star-shaped block copolymers of the structure S—X—(B)n, i.e. one polymer block of polystyrene connected to n polymer blocks of type B (polybutadiene or polyisoprene) through the residue X of a coupling agent. These studies have confirmed that with n greater than or equal to 2 it is possible to achieve morphologies with spherical or cylindrical domains of polystyrene even at styrene contents exceeding 40% by weight. Because the B blocks in such materials cannot serve as bridges connecting glassy polystyrene domains, these polymers have little mechanical strength and poor elastic recovery, and are thus unsuitable as elastomers.